1. Field of the Invention
The present invention relates to a color solid state imaging device using a plurality of color filters.
2. Description of the Prior Art
A two-dimensional solid state imaging device consists of pixels regularly arranged in a horizontal and a vertical directions and, a reading part which reads signal charges after photoelectric conversion stored in the pixels. When the reading part of this two-dimensional solid state imaging device is a CCD (charge-coupled device), the device is called as a CCD type imaging device, while, when the reading part is a MOS scanning circuit, the imaging device is called as a MOS transistor type imaging device. In any type of the device, the signal charges stored in the pixels are read out by two kinds of operations, namely, frame storing operation whereby the signal charges are read out per a frame period as shown in FIG. 6(a) and field storing operation whereby the signal charges are read out per a field period as shown in FIG. 6(b). Since the field storing operation requires half the storage time necessary for the frame storing operation, it is suitable for use in a video movie, etc. to pick up a continuous moving picture. Nevertheless, the field storing operation is not applicable to a still camera or the like obtaining an image of one frame by one shot and therefore, the frame storing operation in combination with an optical shutter is inevitably required for the field storing operation to be used for the above-mentioned usage, i.e., still camera or the like. The frame storing operation is high in vertical resolution without mixing the two pixels in the vertical direction.
Meanwhile, as for single-device coloration in the frame storing operation, a sequential color system among many systems proposed is considered superior because of the following three characteristic features thereof;
(1) the system does not use vertical correlation to obtain luminance signals, thereby realizing high vertical resolution;
(2) color signals are not generated when an object of achromatic color is picked up, so that the generation of a spurious color is restricted; and
(3) the transmittance is high for complementary colors and the sensitivity is good.
An example of the conventional color solid state imaging device of the above-described type is constructed in such an arrangement of color filters as shown in FIG. 7. In FIG. 7, M,G,Cy and Ye represent a magenta filter, a green filter, a cyan filter and a yellow filter, respectively. According to this arrangement of color filters, a magenta color signal (referred to as M hereinbelow)+a green color signal (referred to as G hereinbelow) is obtained in an nH line, while a yellow color signal (referred to as Ye hereinbelow)+a cyan color signal (referred to as Cy hereinbelow) is obtained in an (n+1)H line, for the luminance signals. On the other hand, for the color difference signals, M-G=a red color signal (referred to as R hereinbelow)+a blue color signal (referred to as B hereinbelow)-a green color signal (referred to as G hereinbelow) is obtained in the nH line, and Ye-Cy=R-B is obtained in the (n+1)H line. The above color difference signals M-G and Ye-Cy are alternately obtained per every one horizontal scanning period (referred to as 1H hereinbelow). When an achromatic object is picked up by the color solid state imaging device, if M-G=Ye-Cy=0 is held, it is possible to control the generation of the spurious color.
In another example of the conventional solid state color imaging device, color filters are arranged in such a fashion as shown in FIG. 8. In FIG. 8, M,G,Cy and Ye represent a magenta filter, a green filter, a cyan filter and a yellow filter, respectively. The luminance signal according to this arrangement of the color filters is 1/2(M+G+Ye+Cy) both in the nH line and (n+1)H line. Moreover, two color difference signals 1/2(Ye+M-Cy-G)=R-G/2 and 1/2(Cy+M-Ye-G)=B-G/2 are alternately obtained per every 1H. When an achromatic object is picked up by the conventional apparatus of the second example, if R-G/2=B-G/2=0 is satisfied, it is possible to restrict the spurious color.
In the meantime, it is necessary for the color solid state imaging device that the luminance signals are agreed with each other with good accuracy all over the color region in every horizontal line whichever color the object to be picked up has. Otherwise, striped patterns would be produced per a scanning line on the picked-up image, resulting in the considerable deterioration of the image quality.
In contrast to the above-mentioned necessity, however, the luminance signal M+G in the nH line and that Ye+Cy in the (n+1)H line are respectively the sum of the color signals different from each other in the conventional color solid state imaging device of FIG. 7, and therefore it is considerably difficult to agree the former with the latter all over the color band region.
According to the conventional second example shown in FIG. 8, the luminance signals are the same 1/2(M+G+Ye+Cy) in all of the horizontal lines, without generating no striped patterns per a scanning line. However, in order to arrange the color filters as in the second example, the light receiving area of a pixel of several .mu.m should be divided in half correctly with the accuracy of approximately 1/10 .mu.m to arrange two color filters of different colors. To maintain this arranging accuracy for the whole of the horizontal and vertical 100 pixels is extraordinarily hard. Particularly, if the color filters are formed directly on the light receiving surface of the reading part of the solid state imaging device, it is nearly impossible to maintain the accuracy.
For counterbalancing the aforementioned difficulties in maintaining the manufacturing accuracy, it may be possible to arrange two different color filters for one pixel up and down as illustrated in FIGS. 9(a) and 9(b). More specifically, referring to FIG. 9 wherein Ye,M,Cy and G represent a yellow filer, a magenta filter, a cyan filter and a green filter, a color signal passing to the light receiving part is M*Ye or G*Cy in the nH line, and M*Cy or G+Ye in the (n+1)H line, * being an operator to calculate the product of the color signals passing through the two color filters. Therefore, the luminance signal according to the arrangement of the color filters as shown in FIG. 9 is M*Ye+G*Cy in the nH line, and M*Cy+G*Ye in the (n+1)H line.
The above two luminance signals have the spectral characteristics shown by a broken line in FIGS. 10(c) and 10(d) after the spectral characteristics of M,G,Ye and Cy shown in FIGS. 10(a) and 10(b) are added and multiplied, respectively. As is clear from FIGS. 10(c) and (d), not only the spectral characteristic of the luminance signal M*Ye+G*Cy in the nH line is not agreed with that of the luminance signal M*Cy+G*Ye in the (n+1)H line except in the green band region, but the size of the output signals is reversed in the blue and red band regions. Since this disagreement is impossible to be electrically corrected in spite of that it may be possible to be electrically corrected in the case of spectral compositions having symmetrical shape being arranged in getting out of position, it still remains hard to restrict the generation of the striped patterns in every scanning line.